The present invention relates to a multi-color image forming apparatus such as an electrophotographic color printer/copier for forming a color image by superimposing images of a plurality of colors one on another and, more specifically, to such a multi-color image forming apparatus capable of producing high-quality color images by reducing the degree of a moire phenomenon that may occur when the respective colors are given different screen angles.
To reproduce halftone images, electrophotographic printers/copiers, for instance, employ a dot printing technique in which multi-gradation densities are reproduced in an equivalent manner by varying areas of respective dots. To perform dot printing, an electronically generated screen is generally used. In electrophotographic color printers/copiers that form color images by superimposing images of a plurality of colors one on another, screens are set for the respective colors. In this case, to avoid physical superimposition of dots of the respective colors, the screen angles of the respective colors are made different from each other; that is, screen rotation is effected.
FIG. 7 shows a conventional technique of effecting screen rotation in an electrophotographic color printer/copier.
In the example of FIG. 7, 8-bit digital image data is converted to an analog signal by a D/A converter 1 at clock timings of a pixel clock signal. A comparator 3 compares the resulting analog signal with a triangular-wave signal having a predetermined period that is supplied from a triangular-wave generator 2, so that a pulse-width modulation signal is generated whose width depends on the level of the analog signal. The pulse-width modulation signal serves as a dot signal to be used for generating dots.
In the example of FIG. 7, the frequency of the pixel clock (see FIG. 8(a)) is divided by a 1/2 frequency divider 4 to generate a dot reference clock signal (see FIG. 8(b)) whose period is twice that of the pixel clock signal. The dot reference clock signal is delayed by a delay circuit 5 by integer multiples of the 1/4 period of the dot reference clock signal, to produce 4-phase clock signals T.sub.0 -T.sub.3.
A line counter 7 starts to perform counting on a line signal when a page signal is enabled, and changes outputs Q.sub.1 and Q.sub.0 cyclically in the order of [0, 0], [0, 1], [1, 0] and [1, 1] at each line counting. A selector circuit 6 selects one of the 4-phase clock signals T.sub.0 -T.sub.3 in accordance with the outputs Q.sub.1 and Q.sub.0 of the line counter 7 and signals Color Select-1 and Color Select-0 for discriminating among color signals, and sends a selected clock signal to the triangular-wave generator 2. In this manner, the phase of the triangular-wave signal is shifted at each scanning, to thereby form screen angles of 63.5.degree., -63.5.degree. and .+-.45.degree. (see FIGS. 8 and 9). In the case of 63.5.degree. and -63.5.degree., the phase of the triangular-wave signal is shifted by 90.degree. at each scanning. In the case of .+-.45.degree., the phase of the triangular-wave signal is shifted by 180.degree. at each scanning. The screen angle is 90.degree. when no phase shift is effected. Color images having such screen angles are so combined that the centers of the dots of the respective colors coincide with each other (see FIGS. 10A and 10B and FIGS. 11A and 11B). FIGS. 10A and 10B and FIGS. 11A and 11B show cases where the screen angle is 63.5.degree., -63.5.degree., .+-.45.degree. and 90.degree., respectively.
In the above conventional screen rotation technique, in which the centers of dots of the respective colors coincide with each other, a strong-contrast moire pattern occurs at a pitch at which dots of respective superimposed color images coincide with each other as shown in FIGS. 12A and 12B. In FIGS. 12A and 12B, thick overwritten lines indicate the moire patterns. FIG. 12A shows a case where images having screen angles of 63.5.degree. and -63.5.degree. are superimposed on each other. FIG. 12B shows a case where an image having a screen angle of .+-.45.degree. is further superimposed on the images of FIG. 12A. In this manner, in the conventional screen rotation technique, a strong-contrast moire pattern occurs in parallel with the main scanning direction.
Having a frequency component lower than the screen frequency, this type of moire pattern is easily perceived visually and therefore deteriorates image quality.